The present invention relates to an antenna arrangement for a multi radiator base station antenna, the antenna having a feeding network based on air filled coaxial lines, wherein the coaxial lines are an integrated part of the antenna reflector. The invention especially relates to such a dual polarised antenna having two parallel columns with dual polarised radiators.
Antennas in telecommunication systems such as cellular networks today typically use multi-radiator structures. Such antennas make use of an internal feeding network that distributes the signal to the radiators from a common coaxial connector when the antenna is transmitting and in the opposite direction when the antenna is receiving. Typically radiators are positioned in a vertical column and radiators are fed via a feeding network from a common connector in a single polarisation antenna case, or fed via two feeding networks from two connectors in a dual polarisation case. This vertical column arrangement reduces the elevation beam width of the antenna and increases the antenna gain.
For a single-column antenna, the azimuth beam width is determined by the shape of the reflector and the radiator. Approximately, antenna gain is inversely proportional to the antenna beam width. In order to make a narrow azimuth beam width antenna two or more columns of radiators are typically used. Typical applications are road or railroad sites, or sites that use six sectors instead of the commonly used three sectors. For road and railroad sites, higher antenna gain allows the operator to use a larger distance between sites. A six-sector site can be used to increase the capacity of a cellular network without increasing the number of sites, or to increase the area coverage of a given site by using antennas with higher gain achieved by the narrower azimuth beam width.
Today, cellular antennas often have radiators that can radiate in two orthogonal polarisations. Each polarisation is associated to a feeding network. Thus, two orthogonal channels are created that can be connected to a diversity receiver in the base station. Using diversity reduces fading dips and thus enhances the sensitivity of the receiver. In order for the diversity to be efficient, the signals from the two channels must be sufficiently uncorrelated. Therefore it is necessary to maintain certain isolation between the two channels. For diversity purposes 20 dB isolation is enough, but customers usually specify 30 dB due to filter specification issues in the base station.
For a two-column antenna, the azimuth antenna pattern primarily depends on a complex interaction between the width and shape of the reflector, the radiation pattern of the radiators and the separation between the radiators. It is often difficult to combine high gain with low azimuth side lobe level. Low azimuth side lobe level is important in order to reduce interference from neighbouring sectors.